The generation and release of reactive gases can produce immediate and troublesome problems for the environment, workers, livestock and/or equipment located at the plant, found in the general vicinity of the plant and/or located miles downwind of the plant. When these corrosive gases attack buildings, vehicles, or plants, the results can be devastating. Similarly, when these gases are inhaled or come into contact with human or animal skin, the harmful results are can be quickly noticed and extreme.
A number of treatment methods for reducing or otherwise addressing gas emissions from industrial processes and facilities have been developed through the years including, for example, wet scrubbing in which the targeted compound(s) or particulate(s) are brought into contact with one or more scrubbing solution(s). The scrubbing solution(s) may comprise simply water (for dust) or may comprise or incorporate other reagents or solvents in order to enhance the removal and/or neutralization of the targeted compound(s). Wet scrubbing is particularly effective in removing water soluble toxic and/or corrosive gases like hydrochloric acid (HCl) or ammonia (NH3). The wet scrubber efficiency can typically be improved by increasing residence time in the scrubber and/or by increasing the contact area between the scrubber solution and the treated gas stream.
In a dry or semi-dry scrubbing system, however, the gas stream is not saturated with water. In some cases no moisture is added, while in other cases a minor amount of moisture can be introduced. Accordingly, dry scrubber operations do not generally exhibit the stack steam plume or wastewater handling/disposal requirements associated with wet scrubbing operations. Dry scrubbing systems have been used, for example, for removing acidic gases (such as SO2 and HCl), particularly in connection with treating flue gases. Most dry type scrubbing systems will include both 1) a mechanism for introducing the gas sorbent material into the gas stream and 2) a particulate matter control device for removing reaction products and excess sorbent material as well as any particulate matter already present in the gas stream. Depending on the sorbent media and configuration, dry scrubbing systems can be arranged for handling specific gases including, for example, hydrogen sulfide, mercaptans, aldehydes, thiols, other volatile organic compounds including, for example, dimethyl sulfide and dimethyl disulfide.
Dry scrubbing systems can be arranged for injecting alkaline material(s) (such as hydrated lime and/or soda ash) useful for treating gas streams contaminated with acidic gases such as CO2, SO2 and HCl. The acidic compounds react with the alkaline sorbents to form solid salts, which are then removed in the downstream particulate control device. Higher removal efficiencies (80% or better) can be achieved by increasing the gas stream humidity (i.e., cooling the gas stream with a water spray) without saturating the gas stream. These devices have been widely used on, for example, industrial boilers and municipal waste incinerators.
A third method, and another type of “dry” scrubbing, involves removing a target compound from a gas stream by passing the gas stream through a cartridge or other container that is filled with at least one material selected for its ability to absorb the target compound. Activated carbon, for example, is widely used in compressed air and gas purification to remove oil vapors, odors, and other hydrocarbons from the air. The absorber material has to be replaced or otherwise regenerated after its surface is saturated with the target compound(s).
Wet scrubbers are most commonly used for removing acidic gases from process and/or facility emissions and have been in use for many decades. As detailed above, the wet scrubber uses a liquid—typically water or an aqueous solution—to absorb the acidic gas from the carrier gas stream. When properly configured, the scrubbed gas stream contains residual levels of the acidic gas that do not exceed applicable air quality and worker exposure limits. The scrubbing liquid typically contains a reactive agent that “neutralizes” the absorbed acidic species, thereby producing a scrubber effluent stream that is relatively easily disposed or treated in conventional wastewater treatment plants.
In some instances, however, it is inconvenient or impractical to install a wet scrubber system. In confined, spaces a wet scrubber system may not be practical, and the caustic scrubber solutions can pose material handling hazards. In other instances, the capital expense of a caustic scrubber may also be considerable.
The generation of chlorine dioxide has been the subject of a number of U.S. patents including, for example, U.S. Pat. Nos. 6,635,230; 6,605,558; 6,503,419; 6,458,735; 6,423,289; 6,379,643; 6,174,508; and 5,885,543 to Klatte. These patents disclose various methods for producing chlorine dioxide by activating zeolite crystals that are impregnated with a metal chlorite with protons from a second compound such as acetic acid, phosphoric acid, and citric acid, and/or metal salts such as ferric chloride, ferric sulfate, ZnSO4, ZnCl2, CoSO4, CoCl2, MnSO4, MnCl2, CuSO4, CuCl2, and MgSO4. According to Klatte, the zeolite can be activated by flowing a fluid through a combination of zeolite crystals impregnated with an optional water-retaining substance (to suppress moisture induced outgassing) and sodium chlorite and a second group of zeolite crystals impregnated with a proton generating species.
U.S. Pat. No. 5,776,850 to Klatte, for example, discloses methods for impregnating zeolite crystals with a compound comprising sodium chlorite, acetic acid, citric acid, chlorine, sodium sulfite or sodium bisulfite and the zeolite crystals impregnated according to the disclosed methods.
U.S. Pat. No. 5,853,689 to Klatte, for example, disclose methods for producing chlorine dioxide by activating a mixture of impregnated zeolite crystals, the mixture itself, and a method for regenerating the zeolite crystal mixture after chlorine dioxide production. The activation is accomplished by contacting the zeolite crystal mixture with water, a moisture-containing gas, liquid hydrogen peroxide, liquid sulfuric acid, a ferric chloride solution or a sodium chlorate solution. A zeolite crystal mixture can include zeolite crystals impregnated with sodium chlorite, ferric chloride and/or ferric sulfate or, optionally, calcium chloride. An alternative zeolite crystal mixture can include zeolite crystals impregnated with sodium chlorate, sulfuric acid, a deliquescent and an oxidizer including, for example, hydrogen peroxide, sodium metabisulfite or sodium bisulfite. Klatte teaches that the chlorine dioxide release rate can be controlled by, for example, selecting the concentration and amount of activating liquid, or impregnating the zeolite(s) with a selected weight ratio of one or more of the impregnating compounds.
U.S. Pat. No. 5,730,948 to Matte, for example, discloses methods for producing chlorine dioxide by moving fluid (such as air) through a first bed of zeolite crystals impregnated with an acid and then moving the acid-carrying fluid through a second bed of zeolite crystals impregnated with sodium chlorite and/or chlorine to produce chlorine dioxide. In an optional configuration, some of the chlorine dioxide can be absorbed as it passes through a third bed of chemically impregnated zeolite crystals.
Each of the U.S. patents referenced above is hereby incorporated, in its entirety, by reference, and particularly with reference to the formulation and production of zeolite materials impregnated, doped or loaded with a reactive compound including, for example, alkali metal chlorites.